Wetlands are defined as areas of land that are inundated for at least part
of the year, leading to physico-chemical and biological conditions characteristic
of shallowly flooded systems (IPCC, 1996b). Anaerobic conditions associated
with inundation slow decomposition rates and allow accumulation of large stores
of carbon over long time scales, even in systems of relatively low productivity.
Although wetlands occupy only 4-6 percent of the Earth's land area (~0.53-0.57
Gha) (Matthews and Fung, 1987; Aselmann and Crutzen, 1989), they store an estimated
20-25 percent of the world's soil carbon (350-535 Gt C) (Gorham, 1995). The
rates of carbon accumulation in peats (organic soils commonly associated with
wetlands) vary with age (Armentano and Menges, 1986; Tolonen and Turunen, 1996)
but eventually reach equilibrium when inputs equal losses (slow decomposition
rates applied to very large carbon stores) (Clymo, 1984). Most of the wetland
area and associated carbon storage is in peatlands in temperate and boreal regions;
roughly 10-30 percent is in the tropics.

Decomposition under anaerobic conditions produces methane-a greenhouse gas.
Wetlands are the largest natural source of methane to the atmosphere, emitting
roughly 0.11 Gt CH4 yr-1 of the total of 0.50-0.54 Gt CH4 yr-1 (Fung et al.,
1991). Using a Global Warming Potential (GWP) of 21 for CH4, emissions of ~1.7
g CH4 m-2 yr-1 will offset the CO2 sink equivalent to a 0.1 Mg C ha-1 yr-1 accumulation
of organic matter. The range of CH4 emissions from freshwater wetlands ranges
from 7 to 40 g CH4 m-2 yr-1; carbon accumulation rates range from small losses
up to 0.35 t C ha-1 yr-1 storage (Gorham, 1995; Tolonen and Turunen, 1996; Bergkamp
and Orlando, 1999). Most freshwater wetlands therefore are small net GHG sources
to the atmosphere. Two exceptions are forested upland peats, which may actually
consume small amounts of methane (Moosavi and Crill, 1997) and coastal wetlands,
which do not produce significant amounts of methane (e.g., Magenheimer et
al., 1996). Wetlands appear to be relatively small sources of N2O to the
atmosphere, except when they are converted for agricultural use. Although methane
emissions from wetlands are now reported as part of a nation's GHG emissions,
we compare methane and CO2 emissions equivalently here to demonstrate the impact
of various wetland activities on atmospheric GHGs.
Wetlands are vulnerable to future climate change (see IPCC, 1996b, Chapter 6).
Increased decomposition rates in warmer temperatures-if associated with drier
conditions-may lead to large carbon losses to the atmosphere, particularly from
northern peatlands (Gorham, 1995); warmer temperatures may also lead to enhanced
CH4 emissions. Changes in regional hydrology
caused by precipitation changes may cause loss or new growth of wetlands locally.
Changes in permafrost extent and depth will alter the extent and dynamics of
tundra wetlands. Sea-level rise will impact coastal wetland areas.

Wetlands management takes several forms: conversion to agriculture, drainage
for forestry or agriculture, conversion for urban/industrial land uses, creation
through construction of dams (energy uses), direct harvesting, and wetlands
reconstruction.

Table 4-10 lists major practices that impact wetlands,
along with associated processes affecting carbon storage and methane emission.
Most practices affect CH4 and CO2 emissions in opposite ways. Data for the area
of total wetlands, the area of human-impacted wetlands, and effects on GHGs
are largely unknown for many regions. Hence, Table 4-10
gives qualitative rather than quantitative estimates for the net GHG effect
of different management practices. Drainage of wetlands is associated with potentially
large carbon losses as organic matter that has accumulated slowly over centuries
to millennia is oxidized. Methane emissions from drained wetlands will be reduced
(drained systems may even consume methane), offsetting some of the net GHG emission.
For wetlands that do not emit significant methane (coastal wetlands), carbon
stock changes will dominate. For many freshwater wetlands, methane emissions
roughly balance the effect of carbon stock changes in CO2-equivalent emissions.

Table 4-10: Rates of potential carbon gain under
selected practices for wetland management activity in various regions of the world.

---CH4 (net effect: small increase to small decrease in GHG
emission become reduction of CH4 emissions largely offsets loss of CO2);
loss of biodiversity, increase in flooding, decrease in water quality, increased
availability of food or harvested products

+++CH4 [net GHG effect ~0, where CH4 emissions are large enough to offset
carbon sink to decreased emissions in wetlands where CH4 emissions are small
(especially coastal areas that do not emit significant CH4)]; increase in
water quality, decrease in flooding

f

Creation of new flooded lands

Short term:
-0.1 to -2;
Long term:
0-0.05

>100

Short-term GHG source (in long term may be small sink through sediment
deposition), loss of biodiversity, higher stability of water supply, increased
availability of energy that requires no fossil fuel burning

To calculate the effect of wetlands activities on GHG emissions, information
on areal conversion by wetland type is needed, along with net changes in GHG
emissions. The area of global wetland converted for human use is poorly known,
and estimates are mostly unavailable at the country level. Global estimates
range from 6 percent (Armentano and Menges, 1986) to 50 percent (Moser et
al., 1996), with most conversion in temperate and tropical regions.

Wetland extent and duration of inundation are observable through remote sensing
(e.g., Johnston and Barson, 1993). Carbon accumulation and methane emission
rates can potentially be modeled, although these models are still in developmental
stages for most wetland types. Validation of changes in carbon stores through
field sampling is challenging because of the large size of the pools (e.g.,
the depth of many peaty soils) and the difficulty of physical access. Methane
measurements on a wide scale would be difficult and expensive.

Several non-GHG impacts are associated with land practices that affect wetlands.
Wetlands are specialized habitats that have distinct and often valuable flora
and fauna; their loss is a biodiversity issue. Wetlands sequester many pollutants
at the local level; in several countries, wetlands are constructed to treat
wastewater. They act as a buffer for rapid changes in hydrology, and removal
of wetlands can lead to increased flooding in some areas. Harvested organic
matter from organic soils and peatlands is used as a fuel in some regions. Several
international negotiations pertain to these aspects of wetlands-in particular,
the Convention on Wetlands, the Convention on Biodiversity, and the Marine and
Coastal Work Program (Bergkamp and Orlando, 1999).